(a) Field of the invention:
The present invention relates to a zoom lens system, and more specifically to a zoom lens system consisting of two lens groups for use in compact cameras, single lens reflex cameras and so on.
(b) Description of the prior art:
In the recent years, the progress made in the manufacturing techniques for lenses has made it possible to fabricate lens systems at low cost. This is mainly owing to the reduction of manufacturing cost for lenses realized by the progresses in shaping techniques of plastic lenses and aspherical surfaces.
Further, use of aspherical lenses is now bringing about possibilities to enhance performance of lens systems and reduce numbers of lens components. In order to correct curvature of field favorably in lens systems, it is necessary to minimize Petzval's sums of the lens systems. However, aspherical lenses have no function to correct Petzval's sum. It is therefore impossible to correct curvature of field favorably by using aspherical lenses in lens systems and number of lens components must be increased to correct curvature of field. Accordingly, numbers of lens components have not been reduced sufficiently yet in lens systems.
Furthermore, the zoom lens systems consisting of two lens groups having positive refractive power and negative refractive power respectively, and so adapted as to perform variation of focal length by varying the airspace reserved between these lens groups have a common defect that remarkable distortion is produced by the lens group having the negative refractive power. In case of a zoom lens system consisting of a positive front lens group and a negative rear lens group, for example, remarkable positive distortion is produced at the wide position by the rear lens group having the negative refractive power. In the case of a zoom lens system consisting of a front lens group having negative refractive power and a rear lens group having positive refractive power, on the other hand, remarkable negative distortion is produced at the wide position by the front lens group having the negative refractive power.
In order to correct this distortion, it is necessary to cancel said distortion by producing distortion by the positive lens group so as to minimize the distortion in the lens system as a whole. For this purpose, a surface having a diverging function must be arranged in the lens group having the converging function, and spherical aberration and curvature of field must also be corrected at the same time, thereby inevitably marking it necessary to increase number of lens components constituting the lens group having the positive refractive power.
In order to provide a lens system manufacturable at a low cost, it is necessary to simplify composition of the lens system or reduce number of the lens components thereof and, in addition, correct the aberrations as described above. For this purpose, new technical means must be adopted for lens design. Attention is attracted as this means to use of graded refractive index lenses (hereinafter abbreviated as GRIN lenses). The GRIN lenses are classified into so-called axial GRIN lenses whose refractive indices are varied in the direction along the optical axis and so-called radial GRIN lenses having refractive indices varying in the direction perpendicular to the optical axis. Out of these two types, the radial GRIN lenses are effective for correcting Petzval's sum and used in photographing lens systems. As a photographing lens system using the radial GRIN lens, there is known the lens system disclosed by Japanese Unexamined Published patent application No. 153909/62. However, the lens system disclosed by this patent comprises five or more lens components, and is insufficient in compactness and lightness in weight.